I didn’t found any complaints / solutions regarding to this issue. It’s interesting: In the moment you’re initially copy such a link with umlaut onboxing seems to create a preview. After adding a new line or by posting the preview disappears.
So… basically, everything works? What’s the actual problem?
The markdown parser looks for eligible links for oneboxing and tags these with a special class. In the post preview, such links are turned into oneboxes live, but in posted messages a background job replaces the link with the actual onebox content in the cooked HTML. This job is run asynchronously by sidekiq, so it is possible that the post will become visible in the frontend before the processing job has replaced the onebox.
In first place, after adding the link, the onbox is shown correctly. But after a space or new line, the onbox disappears. And in contrast to this discourse installation (Meta), my one isn’t showing the onbox correctly after posting.